"Duet" continues the macabre apocalyptic story of X, and hightens the feeling of a fast approaching storm. Those who are familiar with Tokyo Babylon will recognize the mention of the Sakurazuka; yet another aspect of the story, which, along with the disturbing imagery of Kamui's visions and the introduction of bizarre genetically manufactured entities by the "bad guys" accelerates the creepiness of the story.
If you like conspiracy theories, darkness, and an approaching apocalypse, this is definately the book for you.
"Duet" is a must have for anyone following the series. My only warning is that some may find Kotori's dream sequence a bit too long, it's about a chapter long, and quite gory.
The book starts out with main character and protagonist Kamui revealing some memories to the reader, and then trying to protect Kotori from Seichiro, a dragon of earth. One also learns abou the past of Nataku, another dragon of earth, and there is a flashback as Fuma is talking to Kamui, about a promice that explains Fuma's loyalty. Meanwhile, Sora and Arashi meet with Grandmother Sumeragi.
Overall, this book spends most time explaining things, such as pasts, what's going on-important, but not necessarily the most exciting. You might no feel as compelled to go back to it once you're done, but if you're into the series, definately get it.